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16 from 4

Port expansion using 74HC595s

16 from 4
16 from 4
Not having enough ports on a microcontroller to drive all the functions of a circuit is a commonly-encountered problem. The clever port expansion boards described here offer a solution, and need just four signals from the microcontroller. The first board provides sixteen digital output bits; the second board, equally simple to drive, can be used to interface to a HD44780-compatible LCD module. A C library is available to facilitate using the boards with AVR microcontrollers.
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Materials

Gerber file

CAM/CAD data for the PCB referred to in this article is available as a Gerber file. Elektor GREEN and GOLD members can exclusively download these files for free as part of their membership. Gerber files allow a PCB to be produced on an appropriate device available locally, or through an online PCB manufacturing service.

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PCB

Extra info / Update

Features
* Port expansion from four outputs to sixteen outputs
* Additional circuit board to interface to HD44780-compatible LCD using four port pins
* Controlled using software SPI emulation with free choice of microcontroller output pins
* Boards may be cascaded
* Maximum total load on all outputs approximately 70 mA
* C library available for AVR microcontrollers to drive port expansion and LCD interface boards

Component list

LCD board

Resistors
R1,R2 = 1kO
R3 = 4kO7
P1 = 10kO preset

Capacitors
C1,C2 = 100nF

Semiconductors
T1 = BC557
IC1 = 74HC595

Miscellaneous
K1 = 2-way pinheader
K2 = 10-way pinheader (2x5)
JP1 = 3-way pinheader with jumper
PCB no. 080682-1

Port expansion board

Resistors
R1 = 1kO

Capacitors
C1,C2 = 100nF

Semiconductors
IC1,IC2 = 74HC595

Miscellaneous
K1,K2 = 2-way pinheader
K3,K4 = 8-way pinheader
K5 = 10-way pinheader (2x5)
JP1 = 3-way pinheader with jumper
PCB no. 080682-2

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